MEMS thermopile IR detector is a typical device in the field of sensing detection and one of the core components for constituting temperature sensor, root-mean-square converter, gas sensor, thermal flow meter and other sensor detectors. Moreover, small size thermopile IR detector can also be used for constructing IR focal plane arrays (FPA) and thus realizing IR imaging. Compared with IR detectors based on other operating principles (such as pyroelectric IR detectors and thermistor IR detectors etc.), thermopile IR detector has obvious comprehensive benefits of measurable constant radiant quantity, having no need to apply bias voltage or use chopper, and being more applicable to mobile applications and field applications. Thus, MEMS thermopile IR detector has an important significance for achieving broader IR detection applications. It has broad civilian and military prospects, and it also has enormous business value and market potential. Research and development of the MEMS thermopile IR detector has become a newly high-tech industry growth point for the 21st century. It can be predicted that MEMS thermopile IR detector will become a more widely used device in many aspects of sensing detection. In particular, as MEMS technology, including device design, manufacturing, packaging, testing and other techniques are becoming increasingly sophisticated, MEMS thermopile IR detector will have a more important position in this field.
Responsivity and detection rate are two important performance parameters for describing IR detector and determining its potential applications in different fields, of which responsivity is the ratio of electrical output and incident IR radiation power, indicating the sensitivity of the IR detector responding to IR radiation. Responsivity affects the value of the detection rate greatly. For a thermopile IR detector, the temperature difference between the hot junctions and cold junctions of the thermocouple strips is an important parameter reflecting the responsivity and detection rate of the device. In order to increase the temperature difference and thereby improve performance of the device, the temperature of the cold junctions is usually maintained to be consistent with that of the substrate. Meanwhile, the hot junctions should be able to effectively transfer the heat absorbed by the IR absorber to the thermocouples. To achieve this effect, realization of heat-conducting structures between cold junctions and substrate as well as between hot junctions and IR absorber is necessary, considering the electrical series connection among thermocouples. The heat-conducting structures shall also have the function of electrical isolation. The reported thermopile IR detectors mainly use their substrates as heat sinks, where the cold junctions of the thermocouples usually overlap directly with the substrates while the hot junctions overlap directly with the absorbers. As the materials of the substrates and the absorbers are conductive to some extent, such a direct overlapping method may influence the output signals of the thermopile IR detectors and ultimately affect device performance.
For thermopile IR detector with fixed structural types (including thermal-conductive-electrical-isolated structures), dimensions, thermocouple materials and other parameters, the values of its responsivity and detection rate shall depend on the absorption efficiency of their IR absorber. In research of IR detectors, silicon nitride films are often used as IR absorbers. However, in the wavelength range of 1-12 μm, the average IR absorption efficiency that can be obtained from silicon nitride is only about 35%. Besides, the thermopile IR detector with silicon nitride-based IR absorber cannot get a high responsivity and detection rate. In view of this, to improve the responsivity and detection rate of IR detectors, it is necessary to increase the absorption efficiency of the IR absorber. In study of the IR detectors over the past few decades, researchers have developed a variety of materials or structures with high absorption and can be used as IR absorbers, among which, gold-black has excellent IR absorption effect due to its nanoscale rough surfaces and is an extremely popular material in research of IR detectors because of its low thermal capacity. When using the material of gold-black as IR absorber, the responsivity and detection rate of the devices can be increased accordingly. However, the preparation of gold-black involves metal evaporation, metallic nano-particle agglutination and other complicated processes. Moreover, its compatibility with the CMOS process is poor and generally can only be produced on structure surfaces after completing process of device structures. In view of this, the mass production of detectors with gold-black as the absorber is limited. ¼-wavelength resonant structures make use of the resonance effect generated when the thickness of the dielectric layer matches the ¼-wavelength of incident IR light to maximize the absorption efficiency of the IR absorber. However, constrained by the resonance condition, detectors using ¼-wavelength resonant structures as absorbers are only sensitive to IR radiation of certain central wavelengths. In addition, the processing requirements for preparation of ¼-wavelength resonant structures are extremely strict. If the dielectric layer thickness and wavelength are slightly unmatched, the IR absorption efficiency will be subject to great attenuation.
Black silicon has a kind of large area forest-like nano-pillar/needle structure. It was once deemed to be a revolutionary new material in electronics industry. Compared with the material of conventional silicon, black silicon has very high absorption efficiency for near-IR light. At present, a variety of methods for preparing black silicon have been proposed, such as high energy femtosecond laser-assisted etching, metal catalysis electrochemical etching and plasma dry etching. Based on comprehensive consideration of processing costs, convenient level of technology, processing compatibility and other aspects, the method of plasma dry etching for preparing black silicon is most commonly used in conventional microfabrication. Researchers have reported the usage of black silicon as the material of IR absorber to improve the performance of thermopile IR detectors: after formation of the basic structure of a thermopile IR detector (including a supporting membrane, thermopile, metallic connection structures and so on), α-Si or Poly-Si layer is deposited on the surface by plasma enhanced chemical vapor deposition (PECVD) technology, high energy ion implantation is carried out, followed by incomplete dry etching. In this way, black silicon can be obtained from the silicon layer and can also be patterned in the IR absorber region. Finally, the device is released. In this method, incomplete etching is adopted for fabricating black silicon. Thus, the controllability over the shapes and sizes of the black silicon is low. For this method, in addition, before the preparation of black silicon, high energy ion implantation is needed for introducing defects in the silicon layer. This thereby increases the complexity of the process. Moreover, this method adopts the technical idea of “black silicon first, followed by structure release”. Thus, it requires strict protection of black silicon from damage during the process of structure release. However, black silicon maintains physical and chemical properties of silicon material to some extent, and is vulnerable to etchant gas destruction during the subsequent XeF2 dry release process; moreover, as the nanostructures in black silicon are with certain height and density, conventional methods such as membrane deposition or coating protection cannot achieve effective protection.